1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device. In particular, it relates to a process of etching a layer of silicon nitride formed on the semiconductor device by immersing the semiconductor device in a hot phosphoric acid solution.
2. Description of the Related Art
In the fabrication of semiconductor devices, a layer of silicon nitride is used as a selective localized oxidation mask or as an insulating layer. This mask must be removed from the semiconductor device, i.e., from a semiconductor substrate on which the mask is formed, and thus the mask is removed by etching. A dry etching has been developed, but such a dry etching is not capable of a selective etching. Accordingly, a wet etching using a hot phosphoric acid solution is preferred when a selective removal of the layer of silicon nitride from a layer of silicone dioxide is desired.
This wet etching is carried out by immersing the semiconductor device, having the layer of silicon nitride formed thereon, in a hot phosphoric acid solution, which is controlled to a boiling condition at a temperature of one hundred and several tens of degrees Centigrade. An evaporation loss of water in the hot phosphoric acid causes the latter to be inspissated, and thus a conventional method utilizes an intermittent supply of a constant volume of a low concentration phosphoric acid solution. The hot phosphoric acid solution is heated to a constant temperature, but the temperature of the hot phosphoric acid solution falls below the boiling point just after the low concentration phosphoric acid solution is supplied, and this temperature drop causes a reduction of the etching rate, and therefore, the etching process takes a longer time than actually needed.
As a result, because the etching time is prolonged, the surface of the silicon dioxide is exposed to the hot phosphoric acid solution after the silicon nitride has been removed, and thus impurities are often deposited on the exposed surface. Accordingly, variations of the quality of the etching may increase, and the manufacturing output thus decreased.
There are known techniques regarding the wet etching using a hot phosphoric acid solution; for example, Japanese Patent Unexamined Patent Publication (Kokai) No. 57-94574 discloses an invention in which the concentration of the phosphoric acid solution is controlled based on the specific gravity of the latter, No. 61-134027 discloses an invention in which water is supplied in correspondence with an evaporated loss, by detecting the liquid level of the phosphoric acid solution, and No. 62-177928 discloses an invention in which a pressure vessel is used to prevent an escape of water in the phosphoric acid solution from the vessel. Nevertheless, these prior arts still suffer from the above-described problems.